Optical signal-to-noise ratio (OSNR) is an important parameter of performance in optical transmission systems, as it suggests a degree of impairment when an optical signal is carried by an optical transmission system that employs optical amplifiers. FIG. 1 illustrates the IEC 61280-2-9 standard definition of an OSNR 101, which defines OSNR as the average between a left OSNR 102 and a right OSNR 103, each defined as the difference in power between the peak power and the noise at half the distance between the peaks.
FIG. 2 illustrates an exemplary device that handles optical signals. Reconfigurable optical add/drop multiplexer (ROADM) 200 includes two arrayed wavelength gratings (AWGs) 201-202 to separate the multiplexed signals in the transmission fiber. In other embodiments, ROADM 200 may include a wavelength selective switch (WSS) to fulfill a similar purpose. ROADM 200 in regular operation may filter out inter-channel amplified spontaneous emission (ASE). As a result, an out-of-band measurement of the OSNR may not truly reflect the actual ASE noise power in the channel. In addition, the bandwidth of the optical signal may almost be as large as the channel filter bandwidth, which may lead to a smooth transition between the noise and the signal. Such smooth transitions may lead to out-of-band OSNR measurements being inaccurate due to the need to have clear separation between the carrier signals.
FIG. 3 illustrates an in-band technique for OSNR measurement, which may be commonly known as “polarization splitting.” Optical signal 300 may contain signal component 301 and a noise component 302. Such an in-band measurement requires the transmitted optical signal 301 to be highly polarized and the ASE noise signal 302 to be randomly polarized. This in-band measurement technique also requires the optical signal to contain only one polarization and that there be a large separation between the optical signal 301 and the ASE noise signal 302 (e.g., at least 10 dB). In such a method, a polarization controller and a polarization splitter are used, as the polarization controller may be used to adjust the polarization of the signal so that all of its power will exit the polarization splitter at one port. As the ASE noise may be randomly polarized (regardless of the state of the polarization controller), approximately half of the ASE noise may exit at one port, while the remaining portion may exit at the other port. However, some devices that use dense wavelength division multiplexing (DWDM) may contain multiple signals, each of which possess a different state of polarization. As a result, this in-band measurement technique might be very time consuming.